Existing document scanners can generally be classified as either flatbed scanners or drum scanners. Flatbed scanners generally have a linear array of detecting devices such as linear charge coupled devices (CCDs). The space between the detecting devices determines the horizontal resolution of the scanning device. The speed that the array is moved along the document and the rate at which the devices are read, determines the vertical resolution of the scanning device. Flatbed scanning devices which have 300 to 600 CCD elements per inch are commercially available.
Drum scanners generally have a single photo-detector and a scanning beam of light. The beam of light scans across a line of the document as the document moves around the drum. The light reflected from the document is directed to the single photodetector. The horizontal resolution of the scanner is primarily determined by the frequency that the output of the single photoreceptor is sampled as the light beam moves across a line of the document.
Drum scanners can also include a linear array of detectors and there are many different types of scanners which combine elements from the above described two general types of scanners.
Scanning a document at a particular resolution (for example at 600 pixels per inch) can introduce patterns into the data. The patterns introduced into data by scanning appear when the scanned data is printed or displayed. Such patterns are frequently referred to as Moiré patterns.
In some instances special patterns are intentionally introduced into an image so that when the image is scanned and then reproduced the Moiré patterns produce visible images. When such images are visible it indicates that a document is not an original, that is, the image was created by scanning an original. Such technology is well developed and is for example described in issued U.S. Pat. No. 5,735,547.
The present technology is directed to a method and apparatus for scanning a document in a manner which changes how and whether Moiré patterns are introduced into an image when the image is scanned.
With the present technology an additional motion is introduced into the scanning operation. This motion is in addition to the motion that is inherent in any document scanning operation. For example, in a flatbed scanner, a linear array of scanning devices is positioned across the document in a first or “x” direction and this array is moved across the document in a perpendicular or “y” direction. In a flatbed scanner the position of the scanning devices are fixed in the “x” direction. In a drum scanner a light beam moves across the document in an “x” direction and the document is moved in the “y” direction. The light reflected from the document is directed to a fixed photoreceptor. With the present technology an additional vibratory motion is introduced into the scanning process. With a flatbed scanner the array of detector devices is moved or vibrated. The vibratory motion can for example be in the “x” direction. With a drum scanner the photoreceptor is moved or vibrated. For example the photodetector can be moved or vibrated in the “x” direction. The amount and direction of the vibratory movement can vary up to about one half the distance between units in the scanner's resolution (i.e. one half a pixel). If one is trying to eliminate Moiré patterns the vibratory movement can be a pseudo random series of movements. If one is trying to introduce a special pattern into the document or to otherwise create special effects, various other type of motion can be used. Movement of the CCD array in a flatbed scanner or movement of the photoreceptor in a drum scanner can be done by a simple piezo electric transducer or by a simple mechanical cam.
With the present technology, the data generated by the vibrated scanning device can be manipulated to generate a digital image similar to that which would be generated by a normal scanning element, except that the digital image so generated does not contain the majority of the Moiré patterns that would have been generated by a normal scanning element. The data from the vibrated scanning element can also be manipulated to enhance certain desired effects. That is, the present technology can be used to enhance patterns that are intentionally placed in an image. For example, an image can be constructed so that it will contain a pattern that will only be visible if the image is scanned with a scanner which is operating in accordance with the present technology.